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| Home > Publications database > Simulation von Erosions- und Depositionsprozessen mehrkomponentiger Oberflächenschichten in Fusionsanlagen |
| Dissertation / PhD Thesis/Book | PreJuSER-58940 |
2007
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/3111
Report No.: Juel-4253
Abstract: The next big step on the way to energy production by means of magnetic confinement fusion will be the Tokamak experiment ITER. The present choice of first wall materials in ITER will unavoidably lead to the formation of mixed carbon, tungsten and Beryllium layers. The impact of these layers on life time of wall components and tritium retention in ITER is unclear. Therefore predictive modelling of erosion processes, impurity transport and deposition processes is important. For this the 3D Monte-Carlo code ERO can be used. Until now, the surface in ERO was described simply as a homogeneous material mixture in a given interaction layer of wall components. In this thesis ERO has been coupled to the existing Monte-Carlo code SDTrimSP to describe material mixing processes in wall components correctly. SDTrimSP describes the surface by calculating the transport of ions in solids. It keeps Crack of the depth dependent material concentration caused by the implantation of projectiles in the solid. The calculation of movements of the recoil atoms within the solid gives reflection coefficients and sputtering yields. Since SDTrimSP does not consider chemical processes a new method has been developed to implement chemical erosion of carbon by the impact of hydrogen projectiles. The new code ERO-SDTrimSP was compared to TEXTOR experiments which were carried out to study the formation of mixed surface layers. In these experiments methane CH4 was injected through drillings in graphite and tungsten spherical limiters into the plasma. A pronounced substrate dependence was observed. The deposition efficiency, i.e. the ratio of the locally deposited to the injected amount of carbon, was 4% for graphite and 0.3% for tungsten. The deposition-dominated area on the graphite limiter covers a five times larger area than on the tungsten limiter. Modelling of this experiment with ERO-SDTrimSP also showed a clear substrate dependence with 2% deposition efficiency for graphite and less than 0.5% for tungsten in good agreement with the experiment. The reason for the substrate dependence is mainly explained by the higher physical sputtering yield of a thin carbon film on top of tungsten substrate compared to graphite substrate. In contrast to the simplifying assumption of homogeneous material mixing in the past, the phenomena of substrate dependence can now be understood with ERO-SDTrimSP. An important result of the comparison between experiment and simulation was that the effective sticking of hydrocarbon radicals hitting the surface must be negligible. Furthermore, it was shown that local re-deposited carbon layers are 10 times more effectively eroded than ordinary graphite. This is explained by the formation of hydrogen-rich carbon layers which are formed by carbon ions with low impact energy. Simulation of the impurity transport in the plasma was checked by comparison with two dimensional light emission distributions of CH and CIII lines in front of the limiters. For explaining the remaining discrepancy in the simulation of deposition on graphite the surface roughness, which is not yet included in the model, might be a key parameter. Now that the simulation Cool ERO-SDTrimSP has been supplied it can be used to model mixed surface layers in fusion facilities and, in particular, it will be used to improve the quality of predictions of erosion and deposition processes in ITER.
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